1. Field of the Invention
This invention relates to a method of manufacturing an electron-emitting device and a method of manufacturing an electron source and image-forming apparatus, using such a method. It also relates to apparatuses to be used for such methods.
2. Related Background Art
There have been known two types of electron-emitting device; the thermoelectron emission type and the cold cathode electron emission type. Of these, the cold cathode emission type refers to devices including field emission type (hereinafter referred to as the FE type) devices, metal/insulation layer/metal type (hereinafter referred to as the MIM type) electron-emitting devices and surface conduction electron-emitting devices.
Examples of FE type device include those proposed by W. P. Dyke & W. W. Dolan, "Field emission", Advance in Electron Physics, 8, 89 (1956) and C. A. Spindt, "PHYSICAL Properties of thin-film field emission cathodes with molybdenum cones", J. Appl. Phys., 47, 5248 (1976).
Examples of MIM device are disclosed in papers including C. A. Mead, "Operation of Tunnel-Emission Devices", J. Appl. Phys., 32, 646 (1961).
Examples of surface conduction electron-emitting device include one proposed by M. I. Elinson, Radio Eng. Electron Phys., 10, 1290 (1965).
A surface conduction electron-emitting device is realized by utilizing the phenomenon that electrons are emitted out of a small thin film formed on a substrate when an electric current is forced to flow in parallel with the film surface. While Elinson et al. proposes the use of SnO.sub.2 thin film for a device of this type, the use of Au thin film is proposed in G. Dittmer: "Thin Solid Films", 9, 317 (1972) whereas the use of In.sub.2 O.sub.3 /SnO.sub.2 thin film and that of carbon thin film are discussed respectively in M. Hartwell and C. G. Fonstad: "IEEE Trans. ED Conf.", 519 (1975) and H. Araki et al.: "Vacuum", Vol. 26, No. 1, p. 22 (1983).
FIG. 20 of the accompanying drawings schematically illustrates a typical surface conduction electron-emitting device proposed by M. Hartwell.
In FIG. 20, reference numeral 1 denotes a substrate and 2 and 3 denote device electrodes. Reference numeral 4 denotes an electroconductive film normally prepared by producing an H-shaped thin metal oxide film by means of sputtering, part of which is subsequently turned into an electron-emitting region when it is subjected to a process of current conduction treatment referred to as "energization forming" as described hereinafter. In FIG. 20, a pair of device electrodes are separated from each other by a distance L of 0.5 to 1 mm and the central area of the electroconductive film has a width W' of 0.1 mm.
Conventionally, an electron emitting region 5 is produced in a surface conduction electron-emitting device by subjecting the electroconductive film 4 of the device to a current conduction treatment which is referred to as "energization forming". In an energization forming process, a constant DC voltage or a slowly rising DC voltage that rises typically at a rate of 1 V/min. is applied to given opposite ends of the electroconductive film 4 to partly destroy, deform or transform the film and produce an electron-emitting region 5 which is electrically highly resistive.
Thus, the electron-emitting region 5 is part of the electroconductive film 4 that typically contains a fissure or fissures therein so that electrons may be emitted from the fissure. Note that, once subjected to an energization forming process, a surface conduction electron-emitting device comes to emit electrons from its electron emitting region 5 whenever an appropriate voltage is applied to the electroconductive film 4 to make an electric current run through the device.
The applicant of the present patent application has proposed a method of manufacturing a surface conduction electron-emitting device having remarkably improved electron-emitting characteristics by forming carbon and/or a carbon compound in an electron-emitting region of the electron-emitting device by means of a novel technique referred to as the activation process. (Japanese Patent Application Laid-Open No. 7-235255.)
The activation process is carried out after the energization forming process. In the activation process, the device is placed in a vacuum vessel, an organic gas containing at least carbon, i.e. an element commonly found in the deposit to be formed on the electron-emitting region in the energization forming step, is introduced into the vacuum vessel and an appropriately selected pulse-shaped voltage is applied to the device electrodes for several to tens of several minutes. As a result of this step, the electron-emitting performance of the electron-emitting device is remarkably improved, that is, the emission current Ie of the device is significantly increased while showing a threshold value relative to the voltage.
Apart from the electron-emitting device, carbonization in a gas, liquid or solid phase is a well known technique for preparing carbonic materials. For carbonization in a gas phase, hydrocarbon gas such as methane, propane or benzene is introduced into a high temperature zone of a processing system and pyrolyzed in a gas phase to produce carbon black, graphite or carbon fiber. As for carbonization in a solid phase, it is known that glassy carbon can be produced from thermosetting resins such as phenol resin and furan resin, cellulose or vinylidene polychloride (M. Inagaki: "Carbonic Material Engineering", Nikkan Kogyo Shinbunsha, pp. 50-80).
However, the activation process is more often than not accompanied by the following problems.
Problem 1: For introducing gas in an activation process, an optimum gas pressure has to be selected and maintained for the gas, although it may be too low to be maintained under control depending on the type of the gas to be used. Additionally, the time required for the activation process can vary significantly or the properties of the substance deposited on the electron-emitting region can be modified remarkably due to the water, hydrogen, oxygen, CO and/or CO.sub.2 existing in the atmosphere of the vacuum chamber, if a vacuum pressure is used. This problem by turn can give rise to deviations in the performance of the electron-emitting devices of an electron source realized by arranging a large number of electron-emitting devices or an image-forming apparatus incorporating such an electron source. Particularly, in the case of a large electron source comprising an electron source substrate, carrying thereon a large number of paired device electrodes, pieces of electroconductive film and wires connecting the electrodes, and a face plate, typically provided with a set of fluorescent bodies arranged vis-a-vis the substrate, with spacers disposed between the electron source substrate and the face plate, to separate them by a distance less than several millimeters and bonded together at high temperature to form a vacuum envelope (referred to as sealing). When a voltage is subsequently applied to the wires of the electrode pairs for energization forming and activation, there arises a problem that it takes a long time for introducing the gas and achieving a constant gas pressure within the envelope in order to compensate the low conductance of the vacuum envelope for gas due to the minute distance between the electron source substrate and the face plate. Thus, there is a demand for a new process that can replace the known activation process using gas. According to a method for producing glassy carbon from cellulose or thermosetting resin proposed in response to this demand, powdery cellulose is dispersed into water, molded by mean of centrifugal force applied thereto, dried, thereafter baked at 500.degree. C. under a pressure of 140 kg/cm.sup.2 and then heated further at 1,300 to 3,000.degree. C. under atmospheric pressure to produce glassy carbon. When cellulose is pyrolyzed, the molded pyrolytic product contains porosities therein, which are then pyrolized as it is heated to above 1,500.degree. C. (M. Inagaki: "Carbonic Material Engineering", Nikkan Kogyo Shinbunsha, pp. 50-80). However, this remarkable phenomenon cannot be applied directly to the activation process for manufacturing a surface conduction electron-emitting device because of the very high temperature and pressure involved. More specifically, as will be described hereinafter, the electroconductive film of the electron-emitting device is made of fine particles that can become agglomerated to lose, totally in some cases, its electric conductivity (because the agglomerated masses of the electroconductive film are electrically isolated which increases the electric resistance of the film). Alternatively or the electron-emitting region of the electroconductive film can become covered with carbon, produced by pyrolysis when the film is heated to a high temperature to increase the device current and hence, the consumption rate of electricity of the image-forming apparatus formed by arranging a large number of such electron-emitting devices.
Problem 2: After the activation process, the gas used for the process, water and other gaseous substances such as oxygen, CO, CO.sub.2 and/or hydrogen are adsorbed by the components of the image-forming apparatus including the face plate carrying thereon a set of fluorescent bodies and the adsorbed gas has to be removed in order to make the apparatus operate stably for electron emission and prevent electric discharges by the residual gas from taking place in the apparatus. While a stabilization process is normally carried out for removing the adsorbed gas by baking the components in a vacuum for a long time at high temperature, such a process has not satisfactorily been able to stabilize the operation of an image-forming apparatus, to date, mainly because the temperature that can be used for the stabilization process is limited depending on the thermal resistance of the components of the electron-emitting devices of an electron source or an image-forming apparatus incorporating such an electron source.
Problem 3: Conventionally, an image-forming apparatus is produced by arranging an electron source substrate carrying thereon a large number of paired device electrodes, pieces of electroconductive film and wires connecting the electrodes and a face plate, typically provided with a set of fluorescent bodies oppositely relative to each other, bonding them together at high temperature to form a vacuum envelope (a step referred to as sealing process), subjecting them to a series of process including an energization forming process and an activation process by applying a voltage to the wires and then testing the electron-emitting and image-forming performance of the apparatus before hermetically sealing the vacuum envelope. Thus, since a number of steps for assembling the image-forming apparatus are conducted after the sealing process, if the electron source substrate is found defective for some reason, the entire image-forming apparatus has to be rejected as a defective product, which consequently increases the average cost of manufacturing image-forming apparatuses.
In view of the above identified problems, there has been a strong demand for a novel method of manufacturing an image-forming apparatus and a manufacturing apparatus to be used with such a method, in which the image-forming apparatus is free from the above problems and the problem of recontamination due to readsorption of water and gaseous substances including oxygen, hydrogen, CO and CO.sub.2 by the degased component is overcome.